Biodegradability (%) ¼ [Methane yield at STP (mL-CH4/g-COD)/
Theoretical methane yield (350 mL-CH4/g-COD)] 100 (1)
Design-Expert (Demo version 7.0, Stat-Ease, Inc., Minneapolis,
MN, USA) was employed for experimental design, modeling, and
graphical display of experimental results.
Production of methane was conducted in 120-mL serum bottles
each having 70-mLof working volume. The medium used to produce
methane contained acidic effluent, and Ni and Co, as well as
the inoculum. Table 2 shows the S/X ratio and concentration of Ni
and Co used in the fermentation. The initial pH of the medium was
adjusted to 7.0 using 2 mol/L of HCl or 2 mol/L of NaOH. The serum
bottles were capped with rubber stoppers and aluminum caps.
They were subsequently flushed with nitrogen gas to ensure
anaerobic conditions. All bottles were incubated at room temperature
(30 ± 2 C) on a shaker operating at 150 rpm. All experiments
were done in triplicates.